As others have mentioned, false negatives and false positives are both annoying problems. The standard diagnostic kit for example comes with a positive control, which frankly is absolutely essential. Synthesizing a primer is trivial, a positive control is not as trivial (especially if it's an RNA sample). It's also an RT PCR, which means it's extremely easy to contaminate samples among each other (so you can never be sure if even one of your samples actually has the virus whether ALL of the samples have it or not). Further, you can't trust regular gel results too much, you need a qRT-PCR cycler which most labs would have but probably not these hackers.

Also were talking about a virulent pathogen sample. Do you truly want to stand in line and collect random snot samples with covid virus without proper training and PPE?